| 摘要 |
PURPOSE:To form fine crystal grains without repeating heating and cooling in the state of subjecting forged and cast steel products to an austenitizing heat treatment by heating these beforehand in such a way that the P value by the specific formula attains a specific range in ferrite range and holding the products at said value. CONSTITUTION:In the stage of heating the needlelike ferrite structure materials of low alloy steel large sized cast and forged products to an austenitizing temp. region, they are held at such heating conditions under which the P of the formula[T: heating and holding temp. ( deg.K), (t): the time for the same (Hr)]in a ferrit region attains 19-21 prior to the austenitizing. Thereby, the crystal grains at the austenitizing heat treatment after this are fined. This is becuase the above- described pretreatment collapses the needlelike characteristic of the needlelike ferrite structure and increases the inclination of the crystal bearings among the adjacent crystal grains (large inclination grain foundary). Here are the places where the nuclei for gamma grains in the stage of austenitizing are produced. In this way, it is estimated that the places for production of the nuclei are increased and the fine formation of the gamma grains is achieved. |
